This invention relates to nuclear reactor fuel assemblies, and in particular to fuel assembly grids for pressurized water nuclear reactors.
Fuel assembly grids for commercial pressurized water nuclear reactors have conventionally been fabricated by welding together a plurality of perpendicularly oriented metal strips at slotted intersections, to form a rigid, egg-crate structure for spacing and supporting nuclear fuel rods passing therethrough. When such fuel assemblies are designed for use in nuclear reactors located in areas subject to high seismic activity, the grids must be strengthened to carry a higher impact load without causing damage to the grid or the fuel rods. Often, such strengthening includes providing larger welds at all the strip intersections. These larger welds not only strengthen the assembly, but have an effect on the coolant flow characteristics through the grid. The larger welds increase the coolant pressure drop across the assembly, since the flow area of each cell defined by the intersecting strips, is reduced at the elevations containing the welds.
Particularly when fuel assemblies of one design are to be inserted into a reactor core containing fuel assemblies of a different design, the grid strength and pressure drop flow characteristics must be compatible. For example, fuel assemblies made from Inconel 718 are considerably stronger than assemblies made from Zircaloy. Although Zircaloy is not as strong as Inconel 718, it absorbs fewer neutrons and accordingly has operating cost advantages. Circumstances can arise where a reactor core containing fuel assemblies of the Inconel 718 type, is to be refueled with a batch of the Zircaloy type, such that after refueling, both types of assemblies will reside in the core. In order to assure that the reactor core can be safely operated at its maxiumum rated power level with the mixed fuel, the fuel supplier must increase the strength of the Zircaloy assemblies while matching the pressure characteristics of the Inconel 718 assemblies.